1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a planar mirror finish, and more particularly to a polishing apparatus for polishing a workpiece by pressing a polishing pad or a grinding plate and the workpiece against each other while moving them in sliding contact with each other.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithography process can form narrower interconnections, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. It is therefore necessary to make the surfaces of semiconductor wafers flat for photolithography. One customary way of flattening the surface of semiconductor wafers has been to polish semiconductor wafers by polishing apparatus.
Heretofore, polishing apparatus for polishing semiconductor wafers comprises a turntable with a polishing pad attached thereto and a top ring for holding a semiconductor wafer to be polished. The top ring which holds a semiconductor wafer to be polished presses the semiconductor wafer against the polishing pad on the turntable. While an abrasive liquid is being supplied to the polishing pad, the top ring and the turntable are rotated about their own axes to polish the surface of the semiconductor wafer to a planar mirror finish.
FIG. 1 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 1, the conventional polishing apparatus comprises a turntable 5 with a polishing pad 6 attached to an upper surface thereof, a top ring 1 for holding a semiconductor wafer 4 which is a workpiece to be polished while rotating and pressing the semiconductor wafer 4 against the polishing pad 6, and an abrasive liquid supply nozzle 9 for supplying an abrasive liquid Q to the polishing pad 6. The upper surface of the polishing pad 6 provides a polishing surface. The top ring 1 is connected to a top ring drive shaft 8, and supports on its lower surface a resilient mat 2 such as of polyurethane or the like. The semiconductor wafer 4 is held on the top ring 1 in contact with the resilient mat 2. The top ring 1 also has a cylindrical guide ring 3 mounted on a lower outer circumferential surface thereof for preventing the semiconductor wafer 4 from being disengaged from the lower surface of the top ring 1 while the semiconductor wafer 4 is being polished. The guide ring 3 is fixed to the top ring 1 against relative movement in the circumferential direction. The guide ring 3 has a lower end projecting downwardly beyond the lower supporting surface of the top ring 1. The guide ring 3 holds the semiconductor wafer 4 on the lower supporting surface of the top ring 1 against dislodgment from the top ring 1 due to frictional forces developed between the semiconductor wafer 4 and the polishing pad 6 while the semiconductor wafer 4 is being polished.
In operation, the semiconductor wafer 4 is held against the lower surface of the resilient mat 2 on the top ring 1, and pressed against the polishing pad 6 by the top ring 1. The turntable 5 and the top ring 1 are rotated about their own axes to move the polishing pad 6 and the semiconductor wafer 4 relatively to each other in sliding contact for thereby polishing the semiconductor wafer 4. At this time, the abrasive liquid Q is supplied from the abrasive liquid supply nozzle 9 to the polishing pad 6. The abrasive liquid Q comprises, for example, an alkaline solution with fine abrasive grain particles suspended therein. Therefore, the semiconductor wafer 4 is polished by both a chemical action of the alkaline solution and a mechanical action of the fine abrasive grain particles. Such a polishing process is referred to as a chemical and mechanical polishing (CMP) process.
Another known polishing apparatus employs a grinding plate made of abrasive grain particles bonded by a synthetic resin for polishing a workpiece. The grinding plate is mounted on the turntable, and an upper surface of the grinding plate provides a polishing surface. Since this polishing apparatus does not employ a soft polishing pad and a slurry-like abrasive liquid, it can polish the workpiece to a highly accurate finish. The polishing process by the grinding plate is also advantageous in that it is less harmful to the environment because it discharges no waste abrasive liquid.
The conventional polishing apparatus shown in FIG. 1 has a spherical bearing 7 positioned between the top ring 1 and the top ring drive shaft 8. The spherical bearing 7 allows the top ring 1 to be tilted quickly with respect to the top ring drive shaft 8 even when the top ring 1 encounters a small slant on the upper surface of the turntable 5. The top ring drive shaft 8 is kept in driving engagement with the top ring 1 by a torque transmission pin 107 on the top ring drive shaft 8 and torque transmission pins 108 on the top ring 1. The torque transmission pins 107, 108 are held in sliding point-to-point contact with each other. When the top ring 1 is tilted with respect to the top ring drive shaft 8, the torque of the top ring drive shaft 8 is smoothly and reliably transmitted to the top ring 1 because the torque transmission pins 107, 108 change their point of contact while transmitting the torque.
The above conventional polishing apparatus are problematic in that while polishing a workpiece, the polishing apparatus suffer large vibrations owing to frictional forces developed between the turntable and the top ring with the workpiece interposed therebetween. An analysis suggests that such large vibrations are caused by a combined action of resistant forces by the rotating top ring and the rotating turntable which are rotated independently of each other, such resistant forces being dependent on frictional forces developed between the surface of the workpiece and the surface of the polishing pad or grinding plate, and restoring forces exerted by the top ring drive shaft and a turntable drive shaft.
When the vibrations become large, the polished surface of the workpiece develops polish irregularities or scratches or other surface damage, and hence the workpiece cannot be polished stably. The vibrations may become so intensive that the workpiece may be forcibly detached from the top ring and no longer will be polished.